1. Technical Field
The present disclosure relates to electrosurgical instruments used for open and endoscopic surgical procedures. More particularly, the present disclosure relates to electrosurgical instruments having an electrode assembly which is designed to disperse or minimize energy concentrations and/or current densities that occur at the junction between insulating material and a conductor, reduce the incidence of flashover during activation and limit thermal spread to adjacent tissue structures.
2. Background
A hemostat or forceps is a simple plier-like tool which uses mechanical action between its jaws to constrict tissue and is commonly used in open surgical procedures to grasp, dissect and/or clamp tissue. Electrosurgical forceps utilize both mechanical clamping action and electrical energy to effect hemostasis by heating the tissue and blood vessels to coagulate, cauterize and/or seal tissue.
By utilizing an electrosurgical forceps, a surgeon can either cauterize, coagulate/desiccate tissue and/or simply reduce or slow bleeding by controlling the intensity, frequency and duration of the electrosurgical energy applied to the tissue. Generally, the electrical configuration of electrosurgical forceps can be categorized in two classifications: 1) monopolar electrosurgical forceps; and 2) bipolar electrosurgical forceps.
Monopolar forceps utilize one active electrode associated with the clamping end effector and a remote patient return electrode or pad which is attached externally to the patient. When the electrosurgical energy is applied, the energy travels from the active electrode, to the surgical site, through the patient and to the return electrode.
Bipolar electrosurgical forceps utilize two generally opposing electrodes which are generally disposed on the inner facing or opposing surfaces of the end effectors which are, in turn, electrically coupled to an electrosurgical generator. Each electrode is charged to a different electric potential. Since tissue is a conductor of electrical energy, when the end effectors are utilized to clamp, grasp, seal and/or cut tissue therebetween, the electrical energy can be selectively transferred through the tissue.
It is known that the process of coagulating small vessels is fundamentally different than vessel sealing. For the purposes herein the term “coagulation” is defined as a process of desiccating tissue wherein the tissue cells are ruptured and dried. The term “vessel sealing” is defined as the process of liquefying the collagen in the tissue so that the tissue cross-links and reforms into a fused mass. Thus, coagulation of small vessels is sufficient to close them, however, larger vessels need to be sealed to assure permanent closure.
With particular respect to vessel sealing, in order to effect a proper seal with larger vessels, two predominant mechanical parameters must be accurately controlled—the pressure applied to the vessel and the gap between the electrodes both of which affect thickness of the sealed vessel. More particularly, accurate application of the pressure is important for several reasons: 1) to oppose the walls of the vessel; 2) to reduce the tissue impedance to a low enough value that allows enough electrosurgical energy through the tissue; 3) to overcome the forces of expansion during tissue heating; and 4) to contribute to the end tissue thickness which is an indication of a good seal. In some instances a fused vessel wall is optimum between 0.001 and 0.006 inches. Below this range, the seal may shred or tear and above this range the lumens may not be properly or effectively sealed.
Using electrosurgical instruments to seal, cut and/or cauterize tissue may result in some degree of so-called “thermal spread” across adjacent tissue structure. For the purposes herein, the term “thermal spread” refers generally to the heat transfer (heat conduction, heat convection or electrical current dissipation) traveling along the periphery of the electrically conductive surfaces. This can also be termed “collateral damage” to adjacent tissue. As can be appreciated, reducing the thermal spread during an electrical procedure reduces the likelihood of unintentional or undesirable collateral damage to surrounding tissue structures which are adjacent to an intended treatment site.
Instruments which include dielectric coatings disposed along the outer surfaces are known and are used to prevent tissue “blanching” at points normal to the activation site. In other words, these coatings are primarily designed to reduce accidental burning of tissue as a result of incidental contact with the outer surfaces end effectors. So far as is known these coating are not designed or intended to reduce collateral tissue damage or thermal spread to adjacent tissue (tissue lying along the tissue plane). Moreover, such coatings are not designed or intended to reduce or displace energy concentrations that can occur at the junction of an insulating material and an active conductor.
Cleaning and sterilizing many of the prior art bipolar instruments is often impractical as electrodes and/or insulation can be damaged. More particularly, electrically insulative materials, such as plastics, can be damaged or compromised by repeated sterilization cycles which may ultimately effect the reliability of the instrument and cause so-called “flashover.” Flashover as used herein relates to a visual anomaly which develops as a result of inconsistent current tracking over the surface of the insulator or insulative coating and/or activation irregularities which may occur when the instrument is repeatedly used during surgery. Put simply, flashover tends to char the surface of the insulate and may effect the life of the instrument and/or the electrode assembly. The effects and industry standards with respect to flashover are discussed in detail in the Annual Book of ASTM Standards, Vol. 10.02, Designations: D495-84; D618; D2303; and D3638.
Firing many of the prior art bipolar instruments is problematic in that energy concentrations and/or heat can be formed at or near the junction between the insulator and an adjacent conductive surface. The energy concentrations may promote inconsistent current trackings or activation irregularities during surgery. Moreover, during repeated use of the instrument, heat can damage or compromise the insulative material of the instrument.